Field of the Invention
The invention relates to a circuit configuration for setting the operating point of a radiofrequency transistor having terminals for supply potentials and also a terminal for a measuring resistor. The invention also relates to an amplifier circuit having a radiofrequency transistor and such a circuit configuration.
Amplifier circuits for radiofrequency signals have a radiofrequency transistor to which the signal to be amplified can be fed at a control input and in which the amplified signal is coupled out at the load circuit. The transistor is held at its operating point by direct current being suitably impressed at the control input and at the load current path.
Amplifier circuits with radiofrequency transistors have different gain properties depending on the supply current. Therefore, resistor networks or active circuits are employed in order to set the operating point of the amplifier transistor as far as possible independently of manufacturing tolerances thereof and the temperature response thereof.
A circuit configuration for setting the operating point for a radiofrequency transistor is, for example, the circuit BCR 400 of Infineon Technologies, as shown for example in the data sheet relating to the integrated circuit BCR 400W dated Feb. 24, 1999. The circuit has two terminals for a supply voltage and also a measurement terminal for connection of a measuring resistor and also a current output for connection to the base of a bipolar radiofrequency transistor. The external measuring resistor connected between measurement terminal and the positive pole of the supply voltage is connected into the collector current path of the radiofrequency transistor. The external measuring resistor has a value of 100 to 220 xcexa9. The voltage drop present across this external resistor has a value of approximately 600 mV. In apparatuses with a low supply voltage, in particular battery-operated apparatuses, this limits the maximum output power of the amplifier circuit. The operating time of a battery-operated apparatus is limited by the not inconsiderable current consumption at the measuring resistor.
The article by J. V. Bellantoni, xe2x80x9cLow Dropout Current Source Protects Silicon MMICs and Transistors,xe2x80x9d RF Design, Cardiff Publishing Co., Englewood Colo., US, Vol. 18, No. 8, Aug. 1, 1995, pages 40, 42 to 44, shows an RF amplifier circuit in which a measuring resistor is connected to the collector terminal of a radio frequency transistor and a regulating circuit is coupled to the base terminal of the transistor. The regulating circuit has a first differential amplifier, which is connected to the measuring resistor, and also a current source, which, on the output side, is formed by a further differential amplifier and is driven by the first differential amplifier.
It is accordingly an object of the invention to provide a circuit configuration for setting the operating point of a radiofrequency transistor which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a circuit that can operate at a low supply voltage and that has a low power consumption. It is a further object to specify an amplifier circuit having such a circuit configuration for setting the operating point.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for setting an operating point of a radiofrequency transistor having a load current path and a control terminal, comprising:
a terminal for a first supply potential and a terminal for a second supply potential;
a measurement terminal for connection to a measuring resistor connected in series with the load current path of the radiofrequency transistor;
an output terminal for connection to the control terminal of the radiofrequency transistor;
a reference voltage source;
a differential element having supply voltage terminals connected to the terminal for the first supply potential and to the terminal for the second supply potential, a first signal input connected to the reference voltage source, and a second signal input connected to the measurement terminal;
a current source connected to and controllable by the differential element and connected to the output terminal;
a first resistor connected to the terminal for the first supply potential and a second resistor connected to the terminal for the first supply potential;
a current source connected to the terminal for the second supply potential;
a current mirror circuit having an input path connected to the current source, an output path connected to the terminal for the first supply potential through the first resistor, and a control line coupling the input path to the output path;
a transistor having a base connected to the control line and an emitter connected through the second resistor to the terminal for the first supply potential and to the measurement terminal.
According to the invention, the object relating to the circuit configuration for setting the operating point is achieved by means of a circuit configuration for setting the operating point of a radiofrequency transistor comprising: a terminal for a first supply potential and a terminal for a second supply potential; a measurement terminal, to which is to be connected a measuring resistor connected in series with the load current path of a radiofrequency transistor; an output terminal, to which a control terminal of the radiofrequency transistor is to be coupled; a differential element, which, in terms of supply voltage, is connected to the terminals for the first and second supply potentials and has a first signal input, which is connected to a reference voltage source, and a second signal input which is connected to the measurement terminal; and a current source which can be controlled by the differential element and is connected to the output terminal; a first current mirror circuit having an input path, which is connected to a current source connected to the terminal for the second supply potential, and having an output path, which is coupled to the input path via a control line and is connected to the terminal for the first supply potential via a first resistor; and a first transistor, whose base is connected to the control line connecting the paths of the first current mirror circuit, and whose emitter is connected via a second resistor to the terminal for the first supply potential and to the measurement terminal.
With the above and other objects in view there is also provided, in accordance with the invention, an amplifier circuit, comprising:
an input terminal for a radiofrequency input signal and an output terminal for a radio frequency output signal;
a circuit configuration as outlined above for setting the operating point of a radiofrequency transistor;
a resistor connected between the terminal for the first supply potential and the measurement terminal of the circuit configuration; and
a radiofrequency transistor having a collector DC-coupled to the measurement terminal and a base DC-coupled to the output terminal of the circuit configuration, the base being coupled to the input terminal for the radiofrequency input signal and the collector being coupled to the output terminal for the radio frequency output signal.
That is, the amplifier circuit having such a circuit configuration additionally has: a resistor, which is connected between the terminal for the first supply potential and the measurement terminal of the circuit configuration, and a radiofrequency transistor, whose collector is DC-coupled to the measurement terminal of the circuit configuration and whose base is DC-coupled to the output terminal of the circuit configuration, whose base is coupled to an input terminal for a radiofrequency input signal and whose collector is coupled to an output terminal for a radio frequency output signal.
The circuit configuration according to the invention has a differential element which is fed by the supply voltage and compares the voltage across the measuring resistor with a reference voltage. Depending on this, the current of a current source is controlled, which current source is to be connected to the control terminal of the radiofrequency transistor, for example the base terminal of a bipolar RF transistor. The difference formation makes is possible to provide a low reference voltage with which the voltage dropped across the external measuring resistor is to be compared. In practice, the reference voltage is approximately 100 mV. The difference between the two voltages controls the current output of the circuit.
A refinement of the invention provides a first current mirror circuit, whose input path is fed by a current source and in whose output path a resistor is connected across which the reference voltage is dropped. The control terminal of a first transistor is connected to the control line connecting the input and output branches of the first current mirror circuit. On the load current side, the measuring resistor is to be connected to the first transistor. The load current of the first transistor is controlled depending on the difference between the voltage across the measuring resistor and the reference voltage generated across the resistor in the first current mirror. An output transistor operated in a common-emitter configuration is driven, in terms of current, via a second current mirror. The load current of said output transistor or second transistor serves for impressing current into the control terminal of the radiofrequency transistor. Instead of a single output transistor, a Darlington circuit comprising at least two transistors is also suitable, which has a higher current driver capability.
The current source in the input branch of the first current mirror circuit is formed in a conventional manner. For this purpose, a transistor is provided whose load current path is connected to reference-ground potential via a resistor and whose control terminal is likewise connected to reference-ground potential via the series circuit of a plurality of diodes. A constant current can be tapped off at the other terminal of the load current path of this transistor.
The first current mirror is realized in a conventional manner by coupling control terminal and load current path of the transistor in the input branch. The control terminal of this transistor is additionally connected to the control terminal of the transistor in the output branch.
The transistors in the first current mirror are bipolar pnp transistors. Likewise, the second transistor forming the differential amplifier and also the current source transistor on the output side are bipolar pnp transistors. The circuit configuration can be realized with MOS transistors instead of bipolar transistors. In this case, npn transistors are to be replaced by n-channel MOS transistors and pnp transistors by p-channel MOS transistors.
The circuit according to the invention has the advantage that only relatively few components are required. The low complexity means that the circuit fits into a small, cost-effective standard housing. The current regulation of the circuit commences at an operating voltage as low as about 2 volts. The signals in the regulating circuit have no phase shift, with the result that the regulation has a high stability. The temperature response of the circuit behaves oppositely to the temperature response of a bipolar amplifier transistor, with the result that overall the amplifier circuit has a largely temperature-stable gain behavior.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for setting the operating point of a radiofrequency transistor and amplifier circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.